Although a turbo compressor using a centrifugal compressor has advantages such as having better efficiency, being lighter in weight and being more stable in operation than a reciprocating compressor, their allowable operating range (i.e., the range of the flow rate to a centrifugal compressor) is limited. At a small flow-rate operating point of a centrifugal compressor (i.e., when the flow rate to a compressor is small), phenomena such as considerable fluid separation at the internal flow field occur, thus causing instable operation phenomena and causing stall and accordingly surge. As a result, rapid decrease in the efficiency and the pressure-ratio of the compressor is caused, the life of the compressor is shortened, and accordingly the compressor is damaged in a short time. To cope with this, various countermeasures are taken to delay instable phenomena such as stall of a compressor, extending a stable operating range.
To extend a stable operating range, a casing treatment is provided in a centrifugal compressor. For example, as in Patent Literatures 1 to 5, at an inner face of a casing surrounding an impeller of a centrifugal compressor are formed a suction ring groove that is located downstream of a leading edge of the impeller and a back-flow ring groove that is located upstream of the leading edge of the impeller. With this configuration, when the flow rate to the centrifugal compressor becomes small, fluid in a channel defined at the inner face of the casing is allowed to flow into the interior of the casing from the suction ring groove, and this fluid is returned to the channel upstream of the leading edge of the impeller from the back-flow ring groove. As a result, the flow rate to the impeller is increased, whereby the operation of the centrifugal compressor becomes stable. In this way, a stable operating range can be extended.